In recent years, nonaqueous electrolyte secondary batteries using metallic lithium, an alloy capable of storing and releasing lithium or a carbon material as the negative active material and a lithium-containing transition metal complex oxide represented by the chemical formula LiMO2 (M indicates a transition metal) as the positive active material have been noted as high-energy-density batteries.
A representing example of the lithium-containing transition metal complex oxide is lithium cobalt oxide (LiCoO2), which has been already put to practical use as the positive active material for nonaqueous electrolyte secondary batteries. For nonaqueous electrolyte secondary batteries using a lithium transition metal oxide, such as lithium cobalt oxide, as the positive active material and a carbon material, such as graphite, as the negative active material, an end-of-charge voltage is generally prescribed at 4.1-4.2 V. In this case, the active material of the positive electrode utilizes only 50-60% of its theoretical capacity. Accordingly, if the end-of-charge voltage is increased to a higher value, a capacity (utilization factor) of the positive electrode can be improved to increase the battery capacity and energy density.
However, the higher end-of-charge voltage renders LiCoO2 more prone to experience structural degradation and increases a tendency of an electrolyte solution to decompose on a surface of the positive electrode. In particular, when the battery is stored in a charged state at a high temperature, a gas generated as a result of a reaction between the positive electrode and the electrolyte solution increases a thickness of the battery, a reaction product increases a resistance and the positive electrode material is cause to disintegrate. These together deteriorate charge-discharge characteristics of the battery, which has been a problem.
To improve high-temperature storage characteristics of the nonaqueous electrolyte secondary batteries using lithium cobalt oxide as the positive active material and a graphite material as the negative active material, various techniques have been proposed heretofore. For example, Patent Literature 1 describes a method wherein a fluorine-substituted aromatic compound or a sulfonyl-containing compound is incorporated in an electrolyte solution.
However, in the case where the end-of-charge voltage of the battery is prescribed at a value (4.3 V or higher) that exceeds a conventional value of 4.2V, even if a fluorine-substituted aromatic compound or a sulfonyl-containing cyclic compound is added to an electrolyte solution, as described in Patent Literature 1, the battery shows a marked deterioration of performance when it is stored in a charged state at high temperatures. Thus, such an attempt has failed to achieve a sufficient improvement.
Patent Literature 1: Japanese Patent Laying-Open No. 2003-203673